Prior art automatic control devices for controlling the quantity of light emitted from a flash tube or flash lamp are shown in FIGS. 1 to 4. The device shown in FIG. 1 is that illustrated in the Figure of U.S. Pat. Re. 26,999 and comprises flash tube circuit A, including flash tube 12 adapted to illuminate the photographic object; light terminating circuit B for terminating the light produced by the flash tube; light measuring circuit C adapted to generate a signal in response to the light reflected from the photographic object; and gating means D for enabling the signal from light measuring circuit C to be input to light terminating circuit B upon excitation of flash tube 12. More specifically, gating means D is intermediate light measuring circuit C and light terminating circuit B. The gating means consists of light activated silicon controlled rectifier (LASCR) 14, which responds to the light being emitted from flash tube 12; and transistor 16 which is adapted to be turned on by means of an electrical signal from light measuring circuit C. In light measuring circuit C, photocell 24 and slidewire resistor 26 constitute the initial detecting circuit for light reflected from the object to be photographed. According to the description in the aforesaid patent, the fast conductivity characteristics of certain light sensitive semi-conductor elements are used such that photocell 24 in and of itself effectively integrates the incident light and converts it into a voltage signal of increasing magnitude appearing at slider 26. The signal at slidewire 26 is amplified by transistors 22, 16. Also, zener diode 18 is responsive to the output of transistor 16 and triggers silicon controlled rectifier (SCR) 20, which, in turn, actuates the remainder of light terminating circuit B. As stated in Re. 26,999, zener diode 18 is necessary because its triggering characteristics are much more accurately controllable than those of SCR 20, which is also necessary because of its power handling capabilities. The sudden conductiveness of SCR 20 short circuits resistor 28 to discharge capacitor 30 causing a trigger pulse to be applied to quench tube 32.
In the aforementioned prior art circuit the use of LASCR 14 coupled with transistor 16, the combination of zener diode 18 and SCR 20, as well as amplifying transistors 22, 16, is unnecessarily complex and the extra components and labor to assemble it results in increased costs in its manufacture. The inventors of the subject application have found that results similar to that obtained by the aforementioned prior art circuit are obtainable with less components, thereby resulting in a less complex circuit and lower maufacturing costs.
Another similar type prior art automatic control device for controlling the quantity of light emitted from a flash tube is disclosed in U.S. Pat. No. 3,517,255 and the automatic light termination circuit of the Figure of that patent is shown in FIG. 2. That circuit comprises flash tube circuit A2, with flash tube 40; light terminating circuit B2 for terminating the energization of flash tube 40; light measuring circuit C2 for receiving the light reflected from a photographic object and generating a signal in response thereto; and resetting switch transistor 42 for enabling a signal from light measuring circuit C2 to be input to light terminating circuit B2 coincidentally with the activation of flash tube 40. Resetting switch transistor 42 is conductive with capacitor 44 charged to energize flash tube 40 and switch 44 open. When switch 44 is closed, flash tube 40 is actuated through capacitor 46, trigger transformer 48, and trigger electrode 50. The closing of switch 44 also lowers the potential at junction 49 which then causes resetting switch transistor 42 to be non-conductive, thereby removing the short circuit across capacitor 51 in the gate circuit of LASCR 52. LASCR 52 measures the light reflected from the photographic object, and capacitor 51 charges the photocurrent generated by LASCR 52. When the voltage of capacitor 51 exceeds the gate trigger voltage of LASCR 52, the latter switches to a conductive state. The conduction of LASCR 52 short circuits resistor 54 to effectively connect the voltage of capacitor 56 across winding 58 of quench trigger transformer 60. Thereby, a trigger pulse is applied to quench tube 62, which fires to short circuit flash tube 40.
However, any chattering of switch 44 during its initial operation may result in a corresponding off-on operation of resetting switch transistor 42, thereby causing an inaccurate actuation of light terminating circuit B2, with the result that flash tube 40 is extinguished before an optimum quantity of light has been emitted. Moreover, the automatic light termination circuit of U.S. Pat. No. 3,517,255 requires resetting switch transistor 42 to control the operation of capacitor 51 and LASCR 52 and actuate light terminating circuit B2.
The following is a description of the circuitry illustrated in FIG. 1 of U.S. Pat. No. 3,519,879, which is similar to that of U.S. Pat. No. 3,517,225, and shown herein as FIG. 3. Flash tube circuit A3, light terminating circuit B3 and light measuring circuit C3 correspond to the same lettered circuits previously described. Tubes 66, 68, and LASCR 64 are not conductive with capacitor 70 charged to its normal operating charge for energizing flash tube 66. The voltage between conductors 72 and 73 provides a base bias on gating transistor 74 to make that transistor conductive, whereby its collector-emitter path effectively short circuits capacitor 76 and resistor 78. The gate of LASCR 64 is thereby held non-conductive and prevented from being actuated by extraneous causes to prevent premature firing of quench tube 68.
Flash tube 66 is fired by the closing of switch 80, which causes the voltage between junction 82 and conductor 73 to drop to zero, removing the base bias from gating transistor 74 to make that transistor non-conductive, and causing the gate of LASCR 64 to be unclamped. Coincident with the aforedescribed operation, light from the photographic object is received by LASCR 64 which causes it to generate a photocurrent proportional to the intensity of the incident light. The photocurrent flows through integration capacitor 76 and anticipation resistor 78. The sum of the integrated voltage of capacitor 76 and the anticipation voltage across resistor 78 effectively appears as a control voltage between the gate and cathode of LASCR 64, in series with the voltage across the lower portion of resistor 84 and across capacitor 86. The increasing control voltage resulting from the light impinging on LASCR 64 eventually exceeds the gate trigger voltage thereof to turn LASCR 64 on, which effectively short circuits resistor 88, thereby dumping the charge on capacitor 90 across winding 92 of quench trigger transformer 94. The resulting light terminating trigger pulse actuates tube 68 to short circuit flash tube 66 thereby terminating the production of light. Diode 96 is necessary to effectively connect the anode-cathode path of LASCR 64 across resistor 88, and capacitor 98 reduces the tendency of LASCR 64 to be triggered into conduction by noise. The circuitry for firing flash tube 66 is conventional.
The firing circuits of FIGS. 1 and 3 of the aforesaid patent are similar, except that the firing circuit of FIG. 3 includes a transformer 100, having a primary winding 102 and secondary winding 104, as illustrated in FIG. 4 herein. Winding 102 is connected between flash tube electrode 104 and conductor 106, such that winding 102 is connected in a series with flash tube 108 between conductors 110 and 106. The gating circuit includes SCR 112, the anode of which is connected to junction 114. The anode of LASCR 64 is connected to the cathode of SCR 112 and the gate of SCR 112 is connected through resistor 118 and rectifier diode 120 to one end of transformer winding 104, the other end of the latter being connected to the cathode of SCR 112. Filter capacitor 122 is connected across the series combination of diode 120 and winding 104, whereby the gate-cathode junction of SCR 112 is connected across the output of the aforementioned filter rectifier circuit which is energized from transformer winding 104.
Prior to the closure of switch 80, flash tube 66 is non-conducting, whereby no current flows through winding 102. Consequently, there is no voltage produced across winding 104 and no voltage applied to the gate-cathode junction of SCR 112, whereby it is non-conductive and LASCR 64 is thereby disabled. Subsequent closure of switch 80 fires flash tube 66, and the large discharge current flowing through the tube also flows through winding 102, which produces a voltage pulse across winding 104 that is rectified by diode 120, filtered by capacitor 122 and applied to the gate-cathode junction of SCR 112. Thus, SCR 112 is made conductive, thereby enabling LASCR 64. The ends of winding 104 are points between which a significant change in voltage occurs and therefore corresponds to the changing voltage between conductors 72 and 73 of the embodiment shown in FIG. 1 of the same patent. SCR 112 is kept conducting for the duration of the flash by appropriate selection of the time constant of capacitor 122 and resistor 120. Subsequent to the termination of the flash, SCR 112 is made non-conductive and LASCR 64 is again disabled, such that SCR 112 performs a gating operation similar to that performed by transistor 74 of the embodiment disclosed in FIG. 1 of the subject patent. The remainder of the control circuitry for LASCR 64, as well as the light terminating circuit B3 are the same as FIG. 1 of the patent.
The inventors of the present invention have discovered that similar results can be obtained by using a different technique that reduces the number of components and complexity of the gating control circuitry disclosed in FIG. 1-3 of U.S. Pat. No. 3,519,879, with a commensurate reduction in manufacturing costs.